Treatment of aluminum and composition therefor



July 19, 1960 I R. J. LIPINSKI A ,778

TREATMENT OF ALUMINUM AND COMPOSITIO INVENTOR:

' RICHARD J. u msm BY WW ATTYS.

United States Patent SITIONTHEREEOR Richard-"J. Eipinskfi-E'rie, Par, assignor to Lord Manufacturing company, Erie', Pa., acorporation of Pennsylvania FiledMan 7; 1957,;S8R1N0. 644,487

9Clhi ms; (Cli 148 6 .24)

The present invention relatesto a novel method of improving the bondability of aluminum surfacestoward organic polymeric materials and to the resulting treated, product; and; more "particularly, the. invention relates toe-novel method whereby" the adhesion of, polymeric organici' materials to aluminum surfaces is enhanced through chemical treatmentofthe metal surface, and to the improved" bond'edt assembly; The invention also relates to novei compositionsfor treating aluminum surfaces; for improving 1 the bondability; thereof" toward or ganic polymeric materials, and to. novelrproducts. comprising the-treated aluminum. The present applicationis a continuation-impart of application Serial. No. 563,9015',

- filed Fehruary-I 1956; now abandoned;

I The; problem; of bonding; organic polymeric materials to aluminum surfacesis becoming of increasing impors tance: The problenris'presently' most prevalent in two mainiriends; ('1jthencoating ofaluminumsurfaces with organio po'lymeric compositions, such as paints and other filims, forprotective and/ or decorative purposes; and 1 21) the application of an organic polymeric adhesive com position -to the aluminum surface in a; bonding operation whereby another material, which maybe organic 01'. inorganic; I is. bonded to the aluminum surface through the agencyof thestated adhesive;

is generally known that the-bonds obtained between organic polymeric materials and aluminum surfaces are not' as greatas is desired in most instances, Although the actual magnitude of; bond strength depends upon the particularmaterial =bonded to the aluminum surface, with any combination means'have been sought by which the adhesion maybe increased. It is to the marked increaseoii adhesion betweenorganic polymeric materials generallyto. aluminum surfaces generallybyan entirely new andt novel"principlethat the present invention is directed.

It -is the principal object of the present inventionto provide'a" method- 0f increasing the; bondability of alnminum surfaces toward organicpolymeric; materials.

It is another object of' the present invention to provide a method ofi bonding'onganicpolymeric materials to aluminum surfaces. by which improved adhesion between the-two stated componentsis obtained.

lt is anotherobject" of thepresent invention to provide w relativelysimple and economic method of treating aluminum surfacesto markedly enhance thebondability of "the-:treated'surface to applied organic polymeric mate rials. k

A; further object of theinvention is to provide novel assemblieszcomprising aluminum surfaces; bonded to; or; ganicpolymeric materials in which the adhesion between the aluminum surface and the organic polymeric-.material'istgreater than inprior assemblies.

Other. objects, including the provision of a; novel} treataluminum surfaces bond moretenaciouslyto; the. treated ed aluminum surface to which organic polymeric mate:

2,945,778 Patented July 19, 1960 mentwithsulfonic acids containing no nitro group,, and

are unexpected in view of a. prior suggested treatment ofaluminum with a solution ofa strong acid containing a nitrosulfbnic acid to serve as an accelerator in dissolv ing the aluminumwherein no adhesion-permitting film waszobtained;

The. method ofthe. present .invention,, therefore oomprilseszwetting the aluminum surface withthe, stated nitrosulfonic acidsolutionuntilithealuminum surface is visibly altered through formation of'fthe stated 'film thereon, and recovering the aluminum. with the film; on the surface thereof. The resulting product" comprises the. altered aluminum surface having thereon a film. constituting a reaction. product between the aluminum. and the nitrosulfonicacid. There, may then be applied to the alumi surface contacted by; the nitrosulfonicacid theid'esired; organic polymeric material. The resultingprod'uct comprises the aluminum surface. and the organic poly: meric materiali bonded to each other through the agency ofgia. film formed on. the aluminum and. constituting a reaction product between the aluminum and the nitrosulfoni'c'ac'nn V i The present invemion'will be more readily understoodfi'om. a consideration-ofithe drawings in which:

I" is" a side view in'section showing a pieoeof aluminumhavingthefilmthereon; V Figs 2' isa side view in section showing; acoating adhered to the aluminum by virtue ofithe. film on the alumimnn surface; and

Fig. 3 is a side,view in.section showing two treated pieces of aliuininurn bonded to each other by means of an afilteSlvd. i

As will appear hereinafter further improvements are obtained Hit including in the nitrosul'fonic. acid. treating solntiomadditives providing, a synergistic actionin combination with the nitrosulfonic acid," principally. boric acidf treatment. of theipresent invention has been found to result inyastly improved bonds between aluminum surfaces amrorganic polymeric materials applied thereto. This means that protective and: decorativecoatings: comprising organic polymeric materialswhichaaretapplied-to such treated aluminum surfaces, adheretheretocmoreatenaciously than the same coatings; applied to; similar: but untreated aluminum surfaces Likewise, adhesivesacomprising polymeric organic materiala appliedtozthe. treated surface so that otherl structurali elements: bondedto the aluminum surface through the agency of'the; applied ad; hesive are more tenacihuslyr bonded: thereto: than: the case with similarrassembliies; produced; however; without the treatment of the invention.

The magnitude. of: the; increase; its adhesion. obtained through the present. invention; may? range; from. several percent to. several .fold the latter, in the case of. aluminumi fll q 11 Ym r $=m te1ia1 systemsiwhichnormallyr have litflfi 0 sis ifi amhonsinsaifinity. foneachother. The reason for the increased adhesibnobtained by the present invention is notpresently fully understood. It. is known; however, i5 1. i 6. it ul onie acidksolutioninitially re,- moves oxideion ithe aluminumsurfaceuand etches. slightly into the surface. More important, however, is the for-' mation of a film which can be visually observed. This film is believed to result from some retention, on the aluminum surface, of the product of the reaction between the nitrosulfonic acid and the aluminum, the aluminum replacing the hydrogen ion of the nitrosulfonic acid and the residue of the nitrosulfonic acid bonding chemically to the aluminum of the aluminum surface. The aluminum atom to which the residue of the nitrosulfonic acid is chemically bonded is itself part of the main mass of metal and is bonded thereto by means of the forces governing its initial incorporation in the surface lattice. The film is thus firmly attached chemically to the aluminum surface. The film also appears to be more readily wetted by organic materials applied thereto than is the untreated aluminum surface. At any rate, organic materials applied to the film adhere thereto, and hence to the metal body, with much greater tenacity than to the same metal untreated.

The treatment is applicable to aluminum, that is to pure or commercially pure aluminum and to alloys comprising aluminum in which aluminum is the major constituent. Examples of alloys which may be treated are: (1) cast aluminum alloys; such as 5-12% silicon, balance aluminum; 3-8% copper and .5-9% silicon, balance aluminum; 3-l0% magnesium, balance aluminum; .4-l% copper and .2-2% magnesium, balance aluminum; .5-10% silicon and .3-4% magnesium, balance aluminum; 2-7% copper and 1-11% zinc, balance aluminum; 34% magnesium and 1-2% zinc, balance aluminum; 1% copper and 1% nickel, balance aluminum; 2% manganese, balance aluminum; 1% copper, 1% nickel and 6.5% tin, balance aluminum; and the like; (2) wrought aluminum alloys; such as 1.2% manganese, balance aluminum; 1.2% manganese and 1% magnesium, balance aluminum; 25% copper, .5-1% manganese and .3-2% magnesium, balance aluminum; 0.8% silicon, 4.5% copper and 0.8% manganese, balance aluminum; 12.2% silicon, 0.9% copper, 1.1% magnesium and 0.9% nickel, balance aluminum; 0.6% silicon, 0.25% copper, 1% magnesium and 0.25% chromium, balance aluminum; 0.4% silicon and 0.7% magnesium, balance aluminum; 1.6% copper, 2.5% magnesium, 0.3% chromium and 5.6% zinc, balance aluminum; and the like.

As far as the nitrosulfonic acid is concerned it has been found that any sulfonic acid containing one or more nitro (NO groups will provide the improved results. Nitrosulfonic acids, as this term is used herein, include any acid having the general formula R(SO H),,(NO where R is any organic radical, preferably an aromatic radical like benzene or naphthalene (including substituted derivative thereof), and n and m are one or more.

Examples of nitrosulfonic acids which may be employed in accordance with the present invention are:

p-Nitrobenzene sulfonic acid, m-Nitrobenzene sulfonic acid, 2-chloro-5-nitrobenzene sulfonic acid, 2,4-dinitrobenzene sulfonic acid, o-Nitroanilinop-sulfonic acid, 2-nitrotoluene-4-sulfonic acid, S-nitrotoluene-Z-sulfonic acid, 2-nitro-bromobenzene-4-sulfonic acid, 2,4-dinitronaphthyl-7-sulfonic acid, 6-amino-4-nitro-1-phenol-2-sulfonic acid, -amino-4-nitro-1-phenol-2-sulfonic acid 3,5-dinitro-p-toluene sulfonic acid, 4,4'-dinitro-2,2'-stilbene disulfonic acid, 1-nitroanthroquinone-S-sulfonic acid.

In treating the aluminum surface with the nitrosulfonic acid in accordance with the present invention, a solution of the nitrosulfonic acid may be applied to the aluminum surface as by brushing, spraying, dipping, or the like. Thenitrosulfonic acids are soluble in water," at least to the extent required for the present invention, and, hence, water represents the preferred solvent medium for use in treating the aluminum surfaces. However, other liquids may be used in place of or in conjunction with water, such as alcohols, like methanol and ethanol; glycols; glycerol; glycol ethers; and the like.

The concentration of the nitrosulfonic acid in the treating solution has been found to be relatively immaterial. In this connection, it has been found that the nitrosulfonic acid, even at very low concentrations, selectively reacts with the aluminum surface. Thus, concentrations of nitrosulfonic acid in the treating solution as low as about 1%, by weight, are satisfactory. The maximum concentration of the nitrosulfonic acid in the treating medium is limited only by the solubility of the particular nitrosulfonic acid in the particular solvent medium selected. In many cases, the concentration of nitrosulfonic acid may go up to as high as 3040%, by weight, although no advantage is to be gained by employing concentrations substantially in excess of about 20%, by weight.

As stated, the pH of the treating medium will be below about 4. In this connection, for general purposes, the lower the pH, that is to say the higher the acidity, the more rapid is the action of the bath on the aluminum and in some cases may go as low as about 0.5. The preferred pH range is between about 1 and about 3.

Generally the nitrosulfonic acid, if used as such, will provide the desired pH. However, in the event that it does not, and especially where a salt of the nitrosulfonic acid is used, nitric acid may be added to the bath to provide. the desired pH. Nitric acid, unlike other strong acids like sulfuric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, and the like, is not detrimental to the formation of the desired film by the nitrosulfonic acid. Thus, as stated above, the bath will be substantially free of strong acid other than nitric acid and, of course, the nitrosulfonic acid. Weak acids, like boric acid, may be included in the bath without detrimental effect on film- -formation, although no advantage is to be gained by employing a weak acid other than boric acid which will be discussed more in detail below. A particularly advantageous synergistic agent in the bath is boric acid either added as such or as a borate in conjunction with sufiicient nitric acid when necessary to provide'the desired pH. The presence of this material has been found to impart a longer useful life to the nitrosulfonic acid bath, to maintain the desired pH conditions in the bath over a longer period of time and to provide an equalizing effect on the treated aluminum surface so that the degree of alteration over the treated surface is more uniform throughout. The concentration of boric acid in the bath may range up to about 10%, by weight, when sufficient nitric acid is present to provide the requisite pH, although generally no advantage is gained by employing amounts above about 5%, by weight. Examples of boric acid-supplying compounds, other than boric acid itself, that may be employed are the'alkali metal borates, especially sodium borate.

It will be apparent from the foregoing that the bath compositions employed in accordance with preferred practice of the present invention may be as follows: (1) a bath consisting essentially of the nitrosulfonic acid; (2) a bath consisting essentially of the nitrosulfonic acid (either added as such or as a salt), and nitric acid; (3) a bath consisting essentially of nitrosulfonic acid and boric acid; (4) a bath consisting essentially of the nitrosulfonic acid (either added as such or as a salt), boric acid (either added as such or as a salt) and nitric acid. By consisting essentially is meant that the bath will not contain material quantities of other materials adversely affecting the function of the bath for the purposes herein intended. As stated previously nitric acid and weak acids like boric acid, are not detrimental to the bath, and, in fact, have a beneficial affect. Other additives, of the type mentioned below may be included in the bath.

A particularly advantageous stable article. of manuassert-2s.

facture adapted for dissolution ina solvent. to. form, a bath is a mixture of a dry, solid, finely-divided nib-105. 1.- fonic acid, likegr2,4-dinitrobenzene sulfo nic acid or 5,-nitroo-toluene sulfonic acid, anddry, solid, finely-divided-borie acid (one bursts) in .weightrati jbetween: about 1.10; 1 and. l0.to.l.

In, anycf the baths orbath making compositions of thepreesnt invention may bev included'anonionic wetting agent to aid: inwetting the aluminum surface, and a complexing. agent to complex. aluminumin. solution and. thereby stabiiizethe; solution over a long period of use. especially advantageous agentpossessing combined wetting and oomplexing properties is the solid condensate 0f"etl'1'yl'ene oxide with a hydrophobicbase formed by condensing propylene oxide with propylene glycol. It has alsobeen found thata small amount of a fluoride not over-5.%,-, by weight, based on the'weight of-thenitro sulfonic acid, has a beneficial effect particularly. where rapid treating. times are desired.

The temperature of the solution during; the, treatment may vary somewhat, depending upon the other conditions, principally pH, and in general, the, higher; the temperature. the greater the rate of reaction. Thus temperature" can be correlated with the other conditions, Pfi cipally pHof' the solution, to provide any, desired rate. of reaction. Broadly speaking, the temperature of the" treating solution may range from roomtemperature up to the boiling point, although in most cases the temperature 'neednot, exceed about 180 F. Preferably, a.temperature above about 100? is employed. 7

. Asstated, treatment of. the aluminum surface with the nitrosulfonic. acid in. accordance with thepresent, invent ionres'ult's in etching of the. surface andthe formation or a film. This film is discernible to the naked eye, although its thickness may be so minute as not to be readily. measurable. During treatment of the aluminum surface a decided, change in the appearance thereof takes place, due in large measuretothe: etchingand formation of the film, and at the outset this change is not uniform over the surface. In time the changed appearance does become uniform, and this uniformity of appearance denotes:substantial'completion ofl'the treatment. Contact, of the nitro-sulfonic acid-containing solution with the aluminum surface may continue. beyond this point, although no significant advantage is obtained. As the film whicnformsbuilds up over-the aluminum surface; it serves as a, barrier, layer to reduce etching to a minimum so that the aluminum can be left in contact with the solution; beyond the. statedpoint. The exact time required to; complete the treatment depends, as will be apparent from theforegoing, upontheacidity of the treatingrsolution, upon the, temperature-of treatment, and, to} some extent, upon the particular nitrosulfonic acid selected. lngvie'w ofthese variables, it is impossible to set numerical time limits, and; at any rate, one becoming familiar with the; process described herein will have no difliculty in determining for any set of conditions a suitable length of" treating time. By suitable adjustment of the conditions the treating time can be as short as a few seconds. In many cases, however, suchshort treating times. are. not necessary or even desirable so that the conditions can be adiusfed to operate in a matter of minutes. Preferably conditions are selected to insure completion of the treatment inlesszthan about mintues.

Af treating the aluminum surface With the nitrosulfdn .a1"d, thefl,sur face1 is dried, pref.erably after rinsing with Watch. The polymeric organic materialmay hen be applied by any conventional means depending upon the exact nature of the polymeric organic material used. The polymericorganic material will generally beof the type thatiwilllform: a film as is conventional in}, protective and/.or decorative coatings and in adhesives. Such materials, include natural and synthetic resins and rubbers, such. as; phenolralde'hyde resins; urea-aldehyde. resins; furfura-l resins; epoxy resins; polyester resins; silicone es ns; nq w nx e s; l ke p lyv ny hl id m w ny acetate, copolymers of vinyl chloride and vinyl acetate", polyvinylidene, and polyvinyl butyral; acrylic, resins; polyurethaneresins and rubbers; polyamide resins; isocyanate polymers and'copoly-mers; polychloroprener polybutadiene; neoprene; Buna-S; Butyl rubben and-the like. The treatment of the present invention produces the most marked results with compositions comprising natuml or synthetic resins, and, 's'uehmaterials constitute the preferr d organic p ym materials employ d-h re h r nic m ial ay pp ed s; fo l t i I dispersion in a suitable liquid solvent ordilujent, or asia pre brm fi m Referringtothe drawings, Figure 1 illustrates a magnifiedcross-section ofa body'ofaluminum 1" the surface of which has been treated in accordance with the pres? ent invention to provide a film 2 constituting a reaction product between the aluminum of thetaluminnmsurface and thenitrosulfonic acid. Figure,2.illustrates.=theprodnot; of. Figure 1 having thereon a layer. 3, oforganiepolymeric material. Figure 3. illustrates. the-vbonding-of the assembly, of Figure 1V to. another solid. body;. in this illustration an assembly similar to that of Figure 1 and consisting of aluminum body 4 having thereon film 5'. Treated aluminum bodies 1' and: 4" are bonded together bymeans of'organicpolymericadhesive 6: Solidbodies which may be 'adhesively bonded to; aluminum as. illusitrateds in Figure 3: may be any. solid structural: load carrying material like metals, resins, rubbers, fabricmlwood, leather, glass, ceramics, and the like.

The present invention will bemore readily understood from a consideration of the following examples which are given forthe purpose of illustration: only and are not intended" to limit the scope: of theinvention inany way:-

Example I In this example, 3 grams;ofparanitrobenzeneisulfonic water and the monoethyl ether of. ethylene glycol. The solution has a pH of about 1, and is maintained at Two strips of aluminum, 1" x 2%." x .064, and twostrip's' of duralumin (4.5% copper, 0.6%,manganese, 1.5%mag'nesium, balance aluminum) of thesame dimensions, are degreased in tri'chloroethyl'ene and wiped with acetone. The stripsfare immersed in the bath for 15 minutes. The; strips are then removed, rinsed; with running water at room temperature dried.-

To, one broad: face: of each of; the strips is. then; ap,- plied. a. curing catalystecontaining epoxy resinprepared by mixing 12 parts of diethylenetriamine with IQO parts of epoxy resin. (The coated strips are allowed to stand openfor about 75 minutes.) Each set of strips isthen brought together with adhesivetherebetween to provide a l z" lap-joint. About 30 psi. pressure isthen applied, and the assemblies areheld for about an hour at"300 F; The assemblies are then removed 7 from theoven and press and allowed to cool, to. room temperature.

'The assemblies are then tested for shear strength, on a Baldwin. tensile tester. The bonded aluminum strips exhibit bondjruptur'e at about,2240 .p. s.i., and the bonded dui alumin strips. exhibit bond" rupture at, about, 1330 psi. Similar, but untreated aluminum and duralumin strips adhered together in the same manner with the same adhesive exhibit bond failure at about 1200- and 9j6 0 p.-s=i, respectively.

Example. 11.

10, 15 and 20 minutes, respectively. The results are as follows:

Time of immersion: Bond strength, p.s.i.

5 minutes 1770 10 minutes 2430 15 minutes 2190 20 minutes 1780 Example III In this example, 10 grams of S-nitro-toluene-Z-sulfonic acid are dissolved in 175 ml. of water, the bath having a pH of about 1.5. Otherwise following the procedure of Example 1, four sets of two strips each of aluminum are immersed in the bath, each set being immersed for a different period of time, namely, 5, 10, 15 and 20 minutes, respectively. The results are as follows:

Time of immersion: Bond strength, p.s.i.

5 minutes 2230 10 minutes 2340 15 minutes 1690 20 minutes 1890 Example IV A solution is prepared by dissolving 10 grams of 2,4- dinitro-1-naphthol-7-sulfonic acid in 175 ml. of water, the bath having a pH of about 1.5. Otherwise following the procedure of Example I, two strips of aluminum are. immersed in the bath for 20 minutes. The resulting bonded strips exhibit bond rupture at about 1410 p.s.i.

Example V A solution is prepared by dissolving 10 grams of mnitrobenzene sulfonic acid in 170 ml. of water, the bath having a pH of about 1. Otherwise following the procedure of Example I, four sets of two strips each of aluminum are immersed in the bath, each set being immersed for a different time, namely 5, 10, 15 and 20 minutes, respectively. The results are as follows:

Time of treatment: Bond strength, p.s.i.

5 minutes 2550 10 minutes 2450 115 minutes 2030 20 minutes 2110 Example VI A bath is prepared by dissolving 10 grams of l-nitroantl1raquinone-$=sulfonic acid in 165 ml. of water. With the bath at 15 F. and having a pH of about 1, and otherwise following the procedure of Example II using aluminum, the results are as follows:

Time of treatment: Bond strength, p.s.i.

minutes 1600 minutes 1700 minutes 1780 minutes Example VII A bath is prepared by dissolving 5 grams of 2,4-dinitrobenzene sulfonic acid in 165 ml. of water. With the bath at 150 F. and having a pH of about 1 and otherwise follfoiilling the procedure of Example II, the results are as o ows:

Time of treatment: Bond strength, p.s.i.

minutes 2390 10 minutes 2140 15- minutes 2170 20 minutes 2220 Example VIII In this example five baths are prepared using 5 grams of the following designated acids in 160 ml. of water (A) benzene sulfonic acid; (B) p-nitrobenzene sulfonic acid; (C) 2,4-dinitrobenzene sulfonic acid; (D) S-nitro-l- 8 anthraquinone-S-sulfonic acid, and (E) 5-amino-4-nitrol-phenol sulfonic acid. 7 V Otherwise following the procedure of Example II and using aluminum strips and immersiontimes of 10, 15 and In this example three baths are prepared by dissolving 3 grams of the following designated acids in a mixture of 88 ml. Cellosolve and 88 ml. of water: (A) benzene sulfonic acids; (B) toluene sulfonic acid, and (C) p-nitro benzene sulfonic acid.

Otherwise following the procedure of Example I with aluminum strips the results are as follows:

Bond strength, p.s.i. Bath A--- 1330 Bath B 1180 Bath C 2240 Example X A bath is prepared by dissolving 4.8 grams of S-nitro-otoluenesulfonic acid in 160 ml. of water. (pH about 1) at F. and an immersion time of one minute, but otherwise following the procedure of Example I, the resulting assembly has a bond strength of 1310 p.s.i.

Example XI Example X is repeated, adding, however, 0.16 gram of sodium acid fluoride to the bath. The resulting assembly has a bond strength of 1600 p.s.i.

Example XII In this example four baths are prepared each containing 10 grams of 4-nitrotoluene-2-sulfonic acid per liter of water, the baths differing as follows: (A) 4-nitrotoluene-2-sulfonic acid only; (B) addition of 5 grams per liter of boric acid; (C) addition of 10 ml. of concentrated nitric acid per liter, and (D) addition of 5 grams of boric acid per liter and 10 ml. of concentrated nitric acid per liter. I I

Using an immersion time of 3 minutes, a bath temperature of F. and an oven cure of C. for 3 hours, but otherwise following the procedure of Example I for aluminum strips, the results are as follows:

Bath: Bond strength,'p.s.i.

Example XIII A bathis prepared by dissolving 30 grams of 2,4-dinitrobenzene sulfonic acid and 5 grams of boric acid in a liter of water. With the bath at 160 R, an immersion time of 5 minutes, permitting the strips coated with adhesive to stand open for 90 minutes before joining, and a curing cycle of 3 hours at 375 C., but otherwise following the procedure of Example I, the assembly (of aluminum strips) exhibits a bond strength of 3090 p.s.i.

Example XIV A bath is prepared by dissolving 15 grams of the sodium salt of 2,4-dinitrobenzene sulfonic acid, 5 grams of boric acid and 15 m1. of concentrated nitric acid in a liter of water. Following the procedure of Example XIII, the assembly (of aluminum strips) exhibits a bond strength of 3178 p.s.i.

With the bath Considerable modification is possible in the selection of the nitrosulfonic acid, and auxiliary agents, if any, and in the amount thereof as well as in the particular technique and procedure employed without departing from the scope of the present invention.

I claim:

1. The method of improving the bondability of an aluminum surface to organic polymeric material which comprises contacting said surface with a solution, at a pH below 4, consisting essentially of a nitrosulfonic acid until said aluminum surface is visibly altered through formation of a film thereon, and recovering the aluminum with said film on the surface thereof.

2. The method of improving the bondability of an aluminum surface toward organic polymeric material which comprises contacting said aluminum surface with a solution, at a pH below 4, consisting essentially of a nitrosulfonic acid and nitric acid, until said aluminum surface is visibly altered through formation of a film thereon and recovering the aluminum with said film on the surface thereof.

3. The method of claim 2 wherein the pH of the solution is between about 0.5 and about 3.

4. The method of improving the bondability of an aluminum surface toward organic polymeric material which comprises contacting said aluminum surface with a solution, at a pH below 4, consisting essentially of a nitrosulfonic acid and boric acid, said solution being substantially free of strong acid other than said nitrosulfonic acid and nitric acid, until said aluminum surface is visibly altered through formation of a film thereon and recovering the aluminum with said film on the surface thereof.

5. The method of improving the bondability of an aluminum surface toward organic polymeric material which comprises contacting said aluminum surface with a solution, at a pH below 4, consisting essentially of a nitrosulfonic acid, boric acid and nitric acid, said solution being substantially free of strong acid other than said nitrosulfonic acid and said nitric acid, until the said aluminum surface is visibly altered through formation of a film thereon, and recovering the aluminum with said film on the surface thereof.

6. A stable composition of matter in solid, finely-divided form, adapted for dissolution in a solvent to provide a bath for treating aluminum surfaces to improve their bondability to organic polymeric materials, consisting essentially of a mixture of a solid, finely-divided nitro sulfonic acid and a solid, finely-divided boric acid compound selected from the group consisting of boric acid and salt thereof.

7. A bath for treating aluminum surfaces to improve their bondability toward organic polymeric material con sisting essentially of a solution having a pH below 4, of a nitrosulfonic acid and boric acid, said solution being sub stantially free of strong acid other than said nitrosulfonic acid and nitric acid.

8. A bath for treating aluminum surfaces to improve their bondability toward organic polymeric material consisting essentially of a solution, having a pH below 4, of a nitrosulfonic acid, boric acid and nitric acid, said solution being substantially free of strong acid other than said nitrosulfonic acid and said nitric acid.

9. The composition of claim 6 wherein the weight ratio of nitrosulfonic acid to boric acid compound is between about 1 to 1 and about 10 to 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,888,457 Gann et a1. Nov. 22, 1932 2,066,842 Lodeesen Jan. 5, 1937 2,172,533 Freeman Sept. 12, 1939 2,637,634 Howard May 5, 1953 2,698,781 Meyer Ian. 4, 1955 

1. THE METHOD OF IMPROVING THE BONDABILITY OF AN ALUMINUM SURFACE TO ORGANIC POLYMERIC MATERIAL WHICH COMPRISES CONSISTING SAID SURFACE WITH A SOLUTION, AT A PH BELOW 4, CONSISTING ESSENTIALLY OF A NITROSULFONIC ACID UNTIL SAID ALUMINUM SURFACE IS VISIBLY ALTERED THROUGH FORMATION OF A FILM THEREON, AND RECOVERING THE ALUMINUM WITH SAID FILM ON THE SURFACE THEREOF. 